Polychlorinated biphenyl (PCB) congeners with multiple ortho chlorine substituents are potent sensitizers of the ryanodine receptor (RyR) and this activity is thought to contribute to the developmental neurotoxicity associated with perinatal PCB exposure. Many of these congeners display axial chirality and are present in industrial PCB mixtures as a racemate (a 1:1 ratio of both atropisomers). However, the ratio of the two enantiomers changes in vivo, probably due to enantioselective processes involving cytochrome (CYP) P450 enzymes. Emerging data suggest significant variability in the enantiomeric enrichment of chiral PCBs in the human population. This, when considered in light of our preliminary data demonstrating that PCB atropisomers differentially sensitize the ryanodine receptor (RyR), raises the question of whether enantiomeric enrichment influences the risk for adverse neurodevelopmental outcomes following PCB exposure. We propose three specific aims to test the hypothesis that chiral PCB congeners undergo enantioselective biotransformation catalyzed by P450 enzymes and that these differences in biotransformation influence neurodevelopmental endpoints. In Aim 1, the enantiospecificity of RyR-mediated mechanisms of developmental neurotoxicity will be characterized in vitro. The effects of pure PCB atropisomers on dendritic morphology will be quantified in primary rat hippocampal neuron-glia co-cultures and correlated with cellular PCB levels. The molecular mechanisms responsible for enantiospecific activation of RyR1 and RyR2 channel complexes will be investigated using biochemical, biophysical and cellular analyses. In Aim 2, the species and isoform-dependent enantioselective binding and metabolism of pure PCB atropisomers by P450 enzymes will be investigated using murine and human microsomes and tissue slices, as well as recombinant human P450 enzymes. The P450 isoforms responsible for the enantioselective metabolism of PCBs in microsomes will be identified using P450 inhibitors. Aim 3 will confirm, in vivo, that metabolism by P450 enzymes is responsible for enantiomeric enrichment of PCBs and that PCB 136 atropisomers cause enantioselective RyR-mediated developmental neurotoxicity. First, a pharmacokinetic model will be developed, in mice, to examine the role of metabolism in the enantioselective disposition of PCBs. Subsequent studies will determine whether perinatal exposure to chiral PCBs causes enantiomeric enrichment-dependent effects on hippocampal expression and function of RyR and dendritic arborization. These studies will make a fundamental contribution to understanding the risk associated with human exposure to chiral PCB congeners that are highly toxic to the developing nervous system and will provide an insight into the role of chirality in the disposition and toxicity of many chiral pollutants, such as pesticides and plasticizers. Given the extensive polymorphism in human CYP genes, the proposed studies may also suggest future investigations into gene-environment interactions that modulate susceptibility to PCB developmental neurotoxicity.